1. Field of the Invention
The present invention relates to walking piezoelectric actuators, and, in particular, to piezoelectric actuators having meshed teeth and a system for driving the actuator in a prescribed stride.
2. Description of Prior Art
A diverse class of piezoelectric actuators relies on frictional traction to forcefully position an object. Scuff-resistant materials generally have moderate non-sliding coefficients of friction. Moderate friction requires the application of a relatively large normal force to achieve a desired tangential, or tractive force. A large normal force requires a rigid container or housing to prevent deformation that otherwise relegates a portion of the actuator's stroke to container strain. Such rigid containers are heavier than the shells in which electromagnetic motors are commonly housed. Electromagnetic motor shells resist motor torque but need not resist large radial forces due to stator sector reactions.
Applicant's Piezoelectric Actuator U.S. Pat. No. 4,928,030 issued May 22, 1990, discloses a stack of actuators which are controlled to perform smooth walking actuation on an object. The normal force is high in this design. The patent discloses a stack of linearly acting actuators acting as lifters, axiers and tangenters for motion in a three axis co-ordinate system.
Applicant's copending application Ser. No. 07/708,643 filed May 31, 1991, now abandoned, discloses a series of actuators which can be activated to turn about an axis rather than move linearly. These actuators may be included in a stack of linearly acting actuators to yield a combination of linear and curved motions in one actuator.
An efficient power amplifier for high-capacitance devices like piezoelectric actuators is disclosed in U.S. Pat. No. 4,628,275 issued Dec. 9, 1986 to Skipper et al. In Skipper a power amplifier behaves as a stable linear D.C. coupled operational amplifier which delivers a large reactive current with only slight losses. A current sensing feedback is utilized to produce a triangular wave form of current in a load string for causing charge to be alternately applied to and removed from the capacitor with any losses replenished from the D.C. power source.
Walking actuators are also activated by a variety of power amplifiers capable of producing non-sinusoidal wave forms appropriate to smooth walking, for example, vacuum tube and solid state class A and A/B power amplifiers, and switching amplifiers such as those taught by Skipper, et al. Class A amplifiers provide instant startup at full motional amplitude, but internally dissipate all of the available power during standby, or quiescent actuator intervals. Efficiencies of these amplifiers, even with negligible quiescent time, are substantially lower than those achieved by non-sinusoidal resonant electric drives.
In Applicant's copending application Ser. No. 07/488,548 filed Mar. 5, 1990 and continued as Ser. No. 07/743,069 filed Aug. 9, 1991, now allowed, electric drive means employing resonance and mechanical Fourier summing of stroke contributions of segmented piezoelectric actuators achieve high efficiency by precluding rubbing during tractive actuator action, or walking. The non-sinusoidal mechanical summed strokes for walking are easily tailored to almost any wave form desired by varying the frequency and amplitude of resonance of each actuator segment. A tracting actuator operates on a continuous traction surface. Advantageously, a continuous surface may be walked by strides of any length within the reach of the actuator, including zero stride.